1. Field of the Invention
The present invention relates to a disk drive that utilizes a composite head to execute a self servo write process to arrange plural sets of servo data in a circumferential direction of a disk, each set containing plural types of servo data each containing a unique type of servo address mark. In particular, the present invention relates to a disk drive suitable for detecting a target type of servo data in the disk as well as a method of detecting a servo address mark in the disk.
2. Description of the Related Art
Magnetic disk drives that use a disk (disk medium) as a recording medium are well known. Magnetic disk drives, notably small-sized magnetic disk drives, commonly use a disk having servo data embedded in a part of a recording surface. The servo data is recorded at equal intervals in a circumferential direction of the disk and radially in a radial direction of the disk. The servo data contains a servo address mark (SAM) that is a code used to determine the servo data.
The use of an exclusive writer provided with a push pin is known as a method of writing servo data in the recording surface of the disk. The servo writer using the push pin mechanically feeds a head (magnetic head). Thus, as track density increases, the servo writer using the push pin is desired to have a very high mechanical accuracy in order to feed the head with more accurate pitches. Further, the servo writer normally writes servo data when the disk is not enclosed in a housing of the magnetic disk drive (that is, when a top cover of the disk drive is open). Thus, a facility such as a clean room is required to allow the servo writer to write servo data.
Thus, in recent years, a servo data writing method called a “self servo write” has frequently been employed which can deal with the increased track density and which does not require facilities such as a clean room. This method is characterized in that the magnetic disk drive itself controls the positioning (tacking) of the head when a servo write is executed on the disk mounted in this apparatus. Various methods have been proposed as the self servo write. One of such methods is to partition servo data into a number of types (groups) in the circumferential direction of the disk in order to carry out head tracking and servo data writes. The servo data is classified into, for example, odd-number sector servo data and even-number sector servo data, or servo data for the apparatus (actual servo data) and servo data used only for the self servo write (virtual servo data). A magnetic disk drive to which this method is applied uses a composite head including a read element and a write element, as a head (magnetic head) used to write or read data in and from the disk. The read element and the write element are offset from each other in a radial direction of the disk.
In an example in which servo data is partitioned into two types including actual servo data and virtual servo data, plural sets of actual servo data and virtual servo data are arranged (recorded) in a circumferential direction of the disk using a servo data write (a self servo write process), described below. It is assumed that actual servo data and virtual servo data have been written in a disk up to a track M. In this condition, the read element of the head is used to read, for example, actual servo data from the track M. On the basis of the read actual servo data, an operation of writing virtual servo data is performed on, for example, a track N. Similarly, an operation of actual servo data is performed using virtual servo data.
In the magnetic disk drive, a signal read from the disk by the head is amplified by a head amplifier and then transmitted to a read channel IC. The signal (read signal) transmitted to the read channel IC is amplified by a variable gain amplifier (VGA). The amplified signal has its waveform shaped by a filter and is then digitalized by an analog/digital converter (ADC). The digitalized signal (read data) is transmitted to a servo address mark (SAM) detector. The servo address mark detector detects, in the read data, a servo address mark contained in servo data. The servo data is detected in the read data on the basis of detection of the servo address mark.
For servo address mark codes contained in servo data, the same codes are normally used regardless of the position (servo position) of the disk at which servo data is recorded. Alternatively, for servo address mark codes, codes that differ from each other by one servo data item may be used at the same radial position of the disk. A magnetic disk drive to which the self servo write method is applied must read only actual servo data or only virtual servo data. Thus, the magnetic disk drive must distinguish actual servo data from virtual servo data. However, if the same servo address marks are applied to both actual and virtual servo data, it is impossible to determine whether actual or virtual servo data is first detected, when a servo address mark is detected. It is thus necessary to add a code for determination to the servo data or use, for determination, a sector code contained in an address code in the servo data.
However, the method of adding a determination code to servo data requires a separate determination code in addition to servo data in the current format. In this case, the servo data becomes longer by the length of the determination code. Thus, the application of the method of adding a determination code reduces an area on the disk (data area) which can be used to record data. Further, it is not ensured that a target type of servo data, i.e. either actual servo data or virtual servo data is detected as a result of the first servo data search. In this case, a servo data search must be repeated until the target type of servo data is found.
On the other hand, with the method of determining the types of servo data using the sector code, the data area is not reduced because the sector code is already contained in address code. However, also with this method, it is not ensured that the target type of servo data is detected as a result of the first servo data search.